The compound represented by Formula (II): (wherein R1 represents a hydrogen atom or a lower alkyl group, and R2 represents a lower alkyl group) is described in, for example, JP 6-56873 A and JP 9-100291 A. These compounds are known to have the ability to enhance movement of the digestive tract.
Methods for preparation of this compound are found in, for example, JP 6-56873 A, Bioorg. & Med. Chem. Lett., vol. 4(11), 1347 (1994) and JP 9-100291 A.
However, the preparation methods found in JP 6-56873 A and Bioorg. & Med. Chem. Lett., vol. 4(11), 1347 (1994) are difficult to implement for industrial purposes because they involve many steps and repeatedly use column chromatography for purification. JP 9-100291 A teaches an improved method for preparing the compound of the present invention, represented by Formula (II): (wherein R1 represents a hydrogen atom or a lower alkyl group, and R2 represents a lower alkyl group), which overcomes the above problems associated with the preparation methods found in JP 6-56873 A and Bioorg. & Med. Chem. Lett., vol. 4(11), 1347 (1994). This publication discloses a method in which a compound corresponding to Compound 5 of the present invention is carbamated in the presence of toluene to prepare a compound corresponding to Compound 6 of the present invention. It also discloses a method in which a compound corresponding to Compound 6 of the present invention is catalytically hydrogenated under a hydrogen atmosphere using a palladium-carbon catalyst to prepare a compound corresponding to Compound 7 of the present invention. Further, it discloses a method in which a compound corresponding to Compound 8 of the present invention is dissolved in methanol together with fumarate and then crystallized by addition of isopropanol to prepare crystals of a fumarate salt of Compound 8 according to the present invention. Furthermore, it discloses a method in which crystals of a fumarate salt of Compound 8 according to the present invention are dissolved in methanol, followed by addition of isopropanol to prepare purified crystals of a fumarate salt of Compound 8 according to the present invention.